The invention relates to a display device comprising an electro-optical display medium between two supporting plates provided with electrodes, a system of picture elements at the area of picture electrodes arranged on a first supporting plate on the side of the display medium arranged in rows and columns, said first supporting plate also being provided with a system of row and column electrodes for presenting selection and data signals by means of which a range of voltages dependent on the electro-optical display medium can be presented across the picture elements for the purpose of picture display, said row electrodes being capacitively coupled to the picture electrodes.
A display device of this type is suitable for displaying, for example, alphanumerical information and video information by means of passive electro-optical display media such as liquid crystals, electrophoretic suspensions, electrochromic materials, etc.
A device of the type mentioned in the opening paragraph is described in "SPIE Vol. 765 Imaging Sensors and Displays" (1987).
A memory action is obtained in this device by using a switching unit (MIM element) per picture element so that the information presented to a driven row remains present across a picture element to a sufficient extend during the period when the other row electrodes are driven; the loss of information (due to leakage currents) is also limited by the capacitive coupling between the row electrodes and the picture electrodes.
However, the use of such display devices in television systems may present problems. In a conventional drive system for television such as the PAL (NTSC) system approximately 575 (525) lines are written during each frame of 1/25 sec. (1/30 sec.), which lines are distributed over an even and an odd field of approximately 288 (265) lines each per 1/50 (1/60) sec. To inhibit degradation of the liquid crystal material it is preferably alternately driven with a negative and a positive voltage across the liquid crystal. For a display screen with approximately 288 (265) lines it is possible to drive the picture cells with the information presented during the odd field period and subsequently drive them with the information presented during the even field period, the voltage across the picture cell during the odd field period having a different polarity than during the even field period. In this case interlacing does not take place but the second picture line is written over the first picture line, the fourth over the third, and so forth. Information of the same polarity presented to a pixel is refreshed every 1/50 sec. (1/60 sec.) and changes polarity. The number of picture lines on the screen is actually only half the total number of lines of the two fields. However, to write a complete picture of 575 (525) lines the picture information must be presented in an interlaced manner so that the information of opposite polarity is not refreshed after 1/50 (1/60) sec. but after 1/25 (1/30) sec. while information of the same polarity is presented every 2/25 (1/15) sec. Since the picture cells are now driven with the same (positive or negative) voltages for a longer time, this information may be partly lost due to the said leakage currents (notably at higher temperatures because the leakage currents then increase). Due to inequalities between positive and negative information flicker may also occur in the picture at a frequency of 25/2 (15) Hz.